Diaphoretickes

Diaphoretickes is a major group of eukaryotic organisms spanning over 400,000 species. The majority of the earth's biomass that carries out photosynthesis belongs to Diaphoretickes. In older classification systems, members of the Diaphoretickes were variously placed in the kingdoms Protozoa or Protista.

Etymology

The name Diaphoretickes derives (''diaforetikés'') meaning diverse, dissimilar, referring to the wide morphological and cellular diversity among members of this clade.

History

Eukaryotes, organisms whose cells contain a nucleus, have been traditionally grouped into four kingdoms: animals, plants, fungi and protists. In the late 20th century, molecular phylogenetic analyses revealed that protists are a paraphyletic assortment of many independent evolutionary lineages or clades, from which animals, fungi and plants evolved. However, the relationships between these clades remained difficult to assess due to technological limitations. Starting in the early 2000s, improvements on phylogenetics allowed the classification of most eukaryotes into a small number of diverse clades called supergroups.

In 2008, a close evolutionary relationship was discovered between some of these clades: Archaeplastida (plants and relatives; sometimes called Plantae), SAR (stramenopiles, alveolates and rhizarians), and two smaller groups of algae, haptophytes and cryptomonads. This collection of organisms contains almost all eukaryotes capable of photosynthesis.

The SAR, haptophytes and cryptomonads were collectively known as chromalveolates or kingdom Chromista due to a hypothesized common ancestor that obtained the ability to photosynthesize, as algae included in them usually contain a unique pigment, chlorophyll ''c''. The relationship between plants and chromalveolates had been described earlier by evolutionary biologist Thomas Cavalier-Smith (1942–2021), who referred to the clade containing both groups as photokaryotes since most of their members are photosynthetic. He later called them corticates, suggesting that they share a common ancestor due to the presence of cortical alveoli (vesicles underneath the cell membrane) in some of their members (glaucophytes and alveolates). However, these names became obsolete, largely due to the discovery that chromalveolates are not monophyletic: these algae evolved the ability to photosynthesize independently from one another.

In 2012, a publication by the International Society of Protistologists (ISOP) established a taxonomic name for this clade, Diaphoretickes, with the following phylogenetic definition:

In the following years, higher quality phylogenetic analyses recovered more protists that fall into this definition (e.g., telonemids, centrohelids, katablepharids), leading to new clades within Diaphoretickes, such as Haptista (centrohelids and haptophytes) and Cryptista (cryptomonads, katablepharids and relatives).

In 2015, Cavalier-Smith and co-authors rejected the name Diaphoretickes proposed by the ISOP, arguing that it was "an entirely unnecessary, and less euphonious third synonym with no intuitive meaning ..which is destabilising and should not be used". Instead, they suggested converting a pre-existing taxonomic name, Corticata, for the clade containing Chromista and Archaeplastida (Plantae). This did not reach consensus, and Diaphoretickes remains widely accepted by the scientific community as the name of this major eukaryotic clade.

Description and diversity

Diaphoretickes is a large clade that includes eukaryotes of very diverse morphologies and cellular structures. Its major groups are: Archaeplastida, organisms with chloroplasts derived from a primary endosymbiosis event (e.g., green algae, plants, red algae, glaucophytes), amounting to at least 450,000 species; the SAR supergroup, which includes the stramenopiles (e.g., diatoms, brown algae, water moulds, more than 100,000 species in total), the alveolates (e.g., ciliates, dinoflagellates, apicomplexans; at least 18,500 species) and the rhizarians (e.g., radiolarians, forams; more than 7,000 combined species); Haptista, composed of haptophyte algae (e.g., coccolithophores) and centrohelid heliozoans; and Cryptista, comprising cryptomonad algae and related protozoa.

A few morphological traits are common to Diaphoretickes members. They are biflagellates or bikonts, meaning their cells typically have two flagella. They include all eukaryotes engaging in photosynthesis, except for the euglenophytes: these are the ochrophytes (among stramenopiles), myzozoans (in alveolates), chlorarachniophytes (in rhizarians), cryptomonads, haptophytes, and most archaeplastids (plants, green algae, red algae and glaucophytes).

Diaphoretickes also includes all amoebae that have axopodia, stiff filaments branching from the cells. These were historically known as Actinopoda, and were divided into the marine radiolaria (rhizarians) and the mostly freshwater heliozoa ("sun animalcules"). The heliozoa are primarily the centrohelids (relatives of haptophytes), actinophryids (stramenopiles) and desmothoracids (rhizarians). There are also some lone heliozoan species such as '' Microheliella maris'', the sister group of Cryptista. Even heliozoa that have not been genetically sequenced are presumed to belong to Diaphoretickes.

Evolution

Evolutionary relationships are still uncertain between the different clades of Diaphoretickes. Haptista and Cryptista, initially hypothesized as relatives of each other (collectively known as the taxon Hacrobia), were later revealed to be more distantly related. In particular, Cryptista and the species '' Microheliella maris'' form a clade known as Pancryptista, which in turn is the closest relative of Archaeplastida, together forming the proposed 'CAM' clade. Telonemia, previously assigned to Hacrobia, is sometimes resolved as the sister clade of the SAR supergroup, forming the hypothesized TSAR clade, while other studies resolve it as more closely related to Haptista. Three small groups of protists—provorans, hemimastigotes, and the species '' Meteora sporadica''—form a clade that may be related to or inside of Diaphoretickes.

The placement of Hemimastigophora is uncertain. Some studies find it nested within the clades of Diaphoretickes, as the best supported hypothesis or a less-supported one. Others find it to be the sister group of all other Diaphoretickes, which under the branch-based definition falls within Diaphoretickes.

The 2024 study gave maximum support to the sister position of ''Meteora'' to Hemimastigophora, and supported also the hypothesis of the new supergroup of Hemimastigophora and ''Meteora'' with Provora (represented only by '' Ancoracysta'' and no nibblerid) outside Diaphoretickes (according to their phylogenetic definition) and so the sister position of the CAM clade and the common clade of SAR, Haptista and Telonemia (with full support to the SAR monophyly, however the TSAR clade was not observed):

Notes

References

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Eukaryote taxa
Diaphoretickes